The invention relates to energizing plasmas with surface waves, and more particularly, it relates to energizing a gas or gases contained in any discharge container that is operated at low pressures as a weakly ionized plasma, such as a fluorescent lamp.
Although fluorescent lamps have been in use for many years and characteristically are relatively efficient, simple, reliable, durable and fast to operate, improvements in any or all of these characteristics are highly desirable.
Some of the principal limitations that prevent improvement of common fluorescent lamps include the requirements for electrodes, for starting circuit, and for a ballast. Electrodes, over time, degrade and contaminate the discharge container's inner surface with debris which causes dimming. Final electrode degregation leads to eventual failure. Both the starting circuit and ballast consume energy which does not contribute to light output. Therefore, the electrodes, ballast and starting circuit contribute to an ordinary fluorescent lamp's inefficiency. Moreover, both the starting circuit and the ballast wear out and eventually fail.
Another limitation of common or ordinary fluorescent lamps is self-absorption which is energy loss due to nonradiative decay within the lamp gas. Self-absorption may be explained by way of description of operation of such a lamp.
An ordinary fluorescent lamp consists of a ballast, starting circuit, and a glass discharge tube containing a mix of argon gas and mercury vapor with electrodes at each end of the tube. Starting a standard fluorescent lamp requires a special electrical circuit which supplies voltage adequate to start the ionization process. Once in operation, the mercury vapor is weakly ionized (1%), and the plasma electrons deliver energy to the un-ionized mercury atoms through collisions. During steady-state operation, the standard fluorescent lamp's ballast prevents current runaway. Power is delivered to the plasma electrons by an electric field generated between the tube electrodes. During the collisions of the electrons with the mercury atoms, the mercury atoms are both excited (i.e., given energy) and ionized. The excited mercury atoms lose their energy both by radiative and nonradiative decay. Most of the radiative decay takes place by the emission of a 2537 Angstrom, U. V. photon. when a U.V. photon of this wavelength interacts with the phosphor on the tube wall, the phosphor converts the U.V. energy to visible light. The energy of the nonradiative decay (de-excitation by electron collisions) of the mercury atoms does not contribute to producing light, and therefore represents a loss of useful energy. The nonradiative decay is principally due to quenching collisions with the plasma electrons. When a 2735 Angstrom U.V. photon is created in the lamp by radiative decay of a mercury atom, it travels a very short distance (&lt;0.2 mm) before it excites and is re-absorbed by another mercury atom. This mercury atom either emits a U.V. photon or loses energy by electron collision.
The emission, re-absorption, and subsequent re-emission of a U.V. photon is repeated hundreds of times before the photon reaches the tube wall and produces light or before the energy initially created is lost by nonradiative decay. This energy loss due to nonradiative decay, will be referred to as energy loss as a consequence of self-absorption. The farther the initial excitation of a mercury atom is from the tube wall, the greater the self-absorption, and hence, the greater the amount of energy loss due to self-absorption. It is therefore an advantage to initially excite the mercury atoms near the discharge tube's inner surface and thereby reduce energy loss due to self-absorbtion.
One way to accomplish this is to deliver the majority of the electric power to the mercury atoms near the discharge tube's inner surface. The delivery of the electric power provides in the sense described above the initial excitation of the mercury atoms. Such an initial excitation condition (i.e., near the discharge tube's inner surface) can be achieved by use of radio frequency surface waves for which electrodes, starting circuits and ballasts are unnecessary. By way of comparison, the ordinary fluorescent lamp delivers the majority of the electric power to the center of the lamp's discharge tube. The ordinary fluorescent lamp also requires electrodes, a ballast, and a starting circuit.
Attempts have been made by others to construct a satisfactory fluorescent lamp that is energized by radio frequency energy, but none have tried surface waves. Much research effort by others has also been expended in energizing a low-pressure weakly-ionized plasma by means of radio frequency surface waves, but not for operation of a lamp, particularly a fluorescent lamp. Despite all of this prior work, no substantial improvements in characteristics of lamp operation have resulted.